SFI - The number of solar flares. Solar flux is measured in solar flux units (SFUs). It is the amount of radio noise or flux emitted at a frequency of 2800 MHz
SN - Sunspot Number
A index - Is an index of geomagnetic activity derived from a scaled average of the previous 24 hours K-index readings
K index - Quantifies disturbances in the horizontal component of the earth's magnetic field with an integer in the range 0-9 with 1 being calm and 5 or more indicating a geomagnetic storm

NOAA - Space Weather Prediction Center - https://www.swpc.noaa.gov/products/station-k-and-indices